Same skeleton, one atom of difference — that is the whole test. Structures drawn live.
1. A Chirality Center Is a Carbon With Four Different Groups
A stereocenter is a carbon whose four bonds go to four distinct groups — single atoms as in bromochlorofluoromethane, or whole chains as at 2-butanol's C2.
2. Scan Every sp³ Carbon and List Its Four Attachments
Walk carbon by carbon and list each sp3 carbon's four substituents: 2-chlorobutane's C2 has four different groups (chiral), while 1-chloropropane's C1 carries two hydrogens (not).
3. Two Identical Groups Means the Carbon Is Not a Stereocenter
Apply this disqualifier first: any CH2 or CH3 is out instantly, and two matching branches — like 2-propanol's twin methyls — cancel any handedness.
4. Rings Count — Compare the Two Paths Around the Ring
For a ring carbon the two "groups" are the two paths around the ring: identical in methylcyclohexane (not chiral), but split by the carbonyl in 3-methylcyclohexanone (chiral).
5. A Molecule Can Have Several Stereocenters — Count Them for 2ⁿ
A molecule with n stereocenters has at most 2n stereoisomers — one for glyceraldehyde, two adjacent for 2,3-dichlorobutane.
6. A Stereocenter Doesn't Guarantee a Chiral Molecule
A meso compound has stereocenters yet is achiral because an internal mirror plane cancels the two halves, so always check the whole molecule before calling it chiral (see meso compounds).
7. Summary
Drop carbons with two identical groups · list four attachments and confirm all differ · treat ring carbons as two paths · count stereocenters for 2n · check for an internal mirror plane. Next: R/S configuration · enantiomers vs. diastereomers.
Worked example
- A chirality centre is a carbon with four different groups.
- C2 bears H, Br, CH3, and –CHBrCH3 — four different groups → a centre. C3 is equivalent → also a centre.
- With two centres there are stereoisomers — and one of them (the R,S) is an achiral meso compound.
Answer. Two chirality centres (C2 and C3).
Quiz yourself
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No. It carries an H, an OH, and two methyl groups. Two identical groups means it fails the four-different-groups test immediately. 2-Butanol, CH3CH(OH)CH2CH3, is the near-twin that is chiral, because a methyl and an ethyl are different.
For a ring carbon the two "groups" are the two paths around the ring. In methylcyclohexane both paths from C1 are identical (CH2-CH2-CH2 either way), so it's not a stereocenter. In 3-methylcyclohexanone the carbonyl breaks the symmetry — one path reaches the C=O sooner than the other — so the paths differ and that carbon is a stereocenter.
No — only three. The 2n formula gives the maximum (here 22 = 4). One of the four is a meso compound: it has an internal mirror plane that makes it superimposable on its reflection, so it is achiral and isn't a separate enantiomer. That collapses the count to three: a pair of enantiomers plus the single meso form.
No. A molecule can contain stereocenters and still be achiral if an internal mirror plane cancels them — that's exactly what a meso compound is. "Has a stereocenter" is about individual carbons; "is chiral" is about whether the whole molecule is non-superimposable on its mirror image. Always check the entire structure before deciding.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.